Working on my stereos lately, I found out that many of mine have this wheel with a belt rubber around it that spins to make the cassette deck functional (see pic below where the screwdriver is pointing to). Either the wheel doesn't turn or the rubber is half gone. Anyone else have this issue and how do you repair such a problem to make the deck fully functional?

This is the "idler drive" found in better quality decks. Basically the motor drives the idler via friction (usually a spring), to whichever spindle is required to move for play /Ffwd /rewind or auto-reverse play. There is usually a small rubber band on the outside of the idler wheel.
These perish and can also become uneven in thickness due the machine not being used for a long time - it presses and indents to the spindle and motor at just two points - which leads to uneven pitch and wow and flutter on playback.
Unfortunately these are very difficult to get as direct replacements, even on high-end cassette decks - so its a case of taking the band off and measuring the diameter and cross-sectional area (usually square) and trying to find a band that is somewhere similar.

Its a regular belt in most cases.
Used as an idler wheel.